The invention relates to a device for determining the level of a medium in a container by means of a capacitive sensor, wherein a voltage source is provided, which subjects the sensor to a measuring frequency, wherein a measuring/evaluating circuit is provided, which ascertains the measured capacitance and compares it with a predetermined capacitance and furnishes the information about the level of the medium in the container.
In capacitive methods for determining the level of a product in a container, the capacitive sensor and container wall form the electrodes of a capacitor. If the container wall is not conductive, then a separate electrode must be provided, inside or outside the container. Between the two electrodesxe2x80x94depending on the level of the medium in the containerxe2x80x94there is either air or the medium, which because of the different dielectric constants of the two substances is expressed as a change in the measured capacitance. Naturally, the measured capacitance is also dependent on the fill level of the medium in the container at the time, since the variables called xe2x80x9clevelxe2x80x9d and xe2x80x9cmeasured capacitancexe2x80x9d both depend functionally on one another. Capacitive sensors can therefore be used both in limit state detection and in continuous level determination.
From German Patent DE 195 36 199 C2, a capacitive level sensor has been disclosed that is mounted at the height of the level to be monitored. In other words, it is a limit state detector, or limit switch. Limit switches are preferentially mounted as overflow suppressors in containers or as means of securing pumps against running empty. If the capacitive sensor is covered by the particular product to be detected, then it has a higher capacitance than in the uncovered state. By means of a capacitance measuring circuit and a comparator, the measured capacitance is compared with a threshold value; from the outcome, it is clearly detectable when the level to be monitored has been reached.
The level measurement by means of capacitive sensors is critical in the sense that the capacitances to be determined and evaluated are on the order of magnitude of 1 pF. In compact limit switches, the relevant capacitances are even less than 1 pF. These very low measurement values are furthermore, in many cases, adulterated by external interference variables. The interference variables involve electromagnetic interference signals, for instance, deposit formation on the sensor, or temperature drift and long-term drifting of the measuring/evaluating circuit.
The object of the invention is to increase the measurement sensitivity of a capacitive sensor and at the same time lessen its sensitivity to interference signals.
This object is attained in that the measuring/evaluating circuit has at least one inductor, which with the measured capacitance forms a series resonant oscillating circuit.
In an advantageous refinement of the device of the invention, the device is a limit state detector, or a limit switch. It is understood, however, that the invention can also be employed in conjunction with a continuous level measurement; in the case of that application, a linearization should subsequently be performed. Preferablyxe2x80x94but without any corresponding limitationxe2x80x94the device of the invention is embodied as a compact sensor.
An advantageous refinement of the device of the invention provides a resistor, which is connected parallel to the inductor of the measuring/evaluating circuit. This resistor serves to damp the series resonant oscillating circuit in a purposeful way.
In an advantageous refinement of the device of the invention, in addition to the measured capacitance, a second capacitor is provided, whose first terminal is connected to the node point to which the measured capacitance and the inductor are connected, and whose second terminal is connected to the internal ground of the measuring/evaluating circuit. This capacitance serves to set the operating point of the measuring/evaluating circuit.
In particular, it is provided that the resistor and the natural resonance of the inductor are dimensioned such that interfering frequencies are largely suppressed. By means of a skilled selection of the inductor, the electromagnetic compatibility behavior of the circuit can be improved substantially. It is also advantageous if the inductor and the capacitance of the second capacitor are dimensioned such that interfering frequencies are largely suppressed.
In a preferred embodiment of the method of the invention, the series resonant oscillating circuit is tuned such that at maximum measured capacitance, the measuring frequency is equal to the resonant frequency, or that the measuring frequency is less than the resonant frequency. Thus when the sensor is uncovered, or when the level of a medium with a low dielectric constant is monitored, a reduction in the total impedance made up of the measured capacitance and the inductor and thus greater sensitivity are already obtained. Since this has no effects on the measuring/evaluating circuit, the absolute temperature drift remains the same, while the relative temperature drift improves. Along with this, the sensitivity of the capacitive sensor to interference variables decreases.
An especially advantageous feature of the device of the invention proposes that the damping of the series resonant oscillating circuit by the resistor be selected such that tolerances in the natural resonance of the inductor and/or the temperature dependency of the inductor are suppressed.